This invention relates to aerodynamic flame holders for airbreathing jet engines. Preferred embodiments include outlet ports in the upper and lower sides of the contour of the flame holder body through which a fuel-air mixture under positive pressure is admitted to the high-velocity air or gas stream wetting the flame holder.
With modern, air-breathing jet engines, thrust augmentation is currently being achieved by means of two different provisions, as follows:
1. Reheating of the turbine exhaust gases in an afterburner
2. Additional combustion of fuel in the cold air stream in bypass ducts (secondary stream).
In both of the above-listed cases the velocity of the gas stream is much too high to sustain stable combustion, and stabilizers must therefore be provided for maintaining combustion. These stabilizers affect the flow of gas around the stabilizers to cause a zone to form behind the stabilizers in which the flow medium recirculates and so induces stable combustion.
Stabilizers of the above-mentioned kind are normally used in the form of rings made of V-shapes (Vee gutter) and are fixedly installed in the combustion chamber space where the afterburning or burning (in the case of combustion in the cold air stream) takes place. However, said stabilizers, together with the fuel injection means needed for the reheating or additional combustion, cause a significant loss in total pressure especially so also at times when reheating or additional combustion cause a significant loss in total pressure especially so also at times when reheating or additional combustion is not needed during certain flight phases. This disadvantageous total pressure loss is all the more significant when one considers that the duration for which afterburners of a flight system are in active use is short as compared with the total flight time of the system.
In order to reduce the loss in total pressure, which ultimately shows up as a loss in performance, so-termed aerodynamic flame holders have been contemplated. These aerodynamic flame holders are of shapes of minimum flow resistance (usually symmetrically shaped) to replace the above-discussed Vee gutters. The function of these flame holders is to blow bleed-air from the high-pressure compressor into the main stream at right or other angles to it whenever reheating or additional combustion is needed. The air issuing from these flame holders causes an umbrella-like shield to form in the main stream which in turn produces a recirculatory zone behind it to promote stable combustion. With this arrangement the fuel needed for combustion must be injected into the recirculatory zone through additional nozzles. This calls for fuel injection means to be installed in the combustion chamber space and these fuel injection means again involve a loss in total pressure, so that the advantage afforded by the flow-promoting contour of the flame holder is partially canceled.
When flame holders such as those described in the preceding paragraph are used, they frequently give trouble from poor ignition in that the mixing process which starts after the fuel has been injected often results in a very incompletely conditioned mixture which is very inhomogeneous and can be lit a few points only. This holds true especially when said flame holders are used in the cold air stream of bypass ducts where the relatively cool air stream impedes the mixing process to an even greater degree. In order to alleviate these mixing troubles a flame holder has been contemplated where a small amount of fuel is already admitted to the stabilizing air within the flame holder, but resort must still be made then to additional fuel injection means outside the flame holder. These additional fuel injection means result in a disadvantageous loss in total pressure as discussed above.
The present invention contemplates providing an aerodynamic flame holder which is simple in design but still ensures reliable ignition of the system and involves no more than a minimal loss in total pressure, or performance, in both the lit and the unlit operating conditions.
This invention more particularly contemplates providing a flame holder where the entire fuel needed for combustion in the afterburner or in the bypass stream is internally mixed with air in a mixing chamber enclosed by the flame holder and is then admitted to the outside gas stream in a condition already premixed with air and processed for ready ignition. This supply of all necessary fuel by way of the flame holder mixing chamber obviates the need for fuel injection means outside the flame holder and so eliminates the attendant loss in total pressure or performance in both the lit and the unlit operating conditions. Since the invention provides a fully integrated system of flame holder, fuel injection means and mixing chamber, it enables the flame holder to be contoured for optimum flow for maximum benefit to the overall system aerodynamically.
With the flame holder of the present invention the fuel needed for combustion is admixed not only partly but wholly to the stabilizing air already before it issues from the flame holder. In this process the fuel is not simply injected into the stabilizing air stream but after the fuel has been admitted the fuel-air mixture is first conditioned in a separate mixing chamber where it turns into an ignitible, homogeneous blend. This mixing chamber advantageously ensures that sufficient time is allowed for fully conditioning the fuel-air mixture for ignition and ensures that fuel can be swirled and thoroughly blended as needed for homogeneous distribution throughout the mixture.
Owing to the fact that the mixture issuing from the mixing chamber through the outlet ports has already been conditioned for ready ignition, ignition of the mixture will reliably take place whenever needed.
This invention also provides a further advantage over conventional, not fully integrated systems in that it reduces the overall length of the afterburner since the previously known systems having fuel injection means outside the flame holder require a certain minimum distance between fuel injection means and flame holder for conditioning the fuel-air mixture. Also, by eliminating external fuel injection means which interfere with the main stream the flame holder of the present invention provides an additional advantage in that the distribution of the issuing ignitible mixture can be optimally adapted to the needs of the respective engine and that the temperature profile attending combustion can be largely controlled by suitably selecting the geometry, distribution and orientation of the outlet ports.
A further advantage provided by the present invention is that the amount of compressed air needed for a certain depth of penetration of the stabilizing medium into the main stream of gas is reduced in keeping with the fuel content in the stabilizing air. Still another advantage is that the pressure level of the stabilizing air used in the process can be relatively low, so that the air needed can be drawn from the low or intermediate pressure portion of the compressor. The two last cited advantages are of great importance considering that the amount and pressure level of the air needed for stabilizing the flame are often exceedingly important if not determinant criteria in practical applications. That is, the efficiency of the total engine system is significantly improved by reducing the pressure requirements for the stabilizing air.
In preferred embodiments of the present invention the mixing chamber is constructed as a hole or hollow space which extends at right angles to the direction of the main flow and over the entire span of the flame holder. One or more lateral inlet ducts leading to said hollow space are provided. Each inlet duct is provided with an air feed line and a fuel line such that the fuel is atomized and initially admixed to the feed air while still in the inlet duct. The actual conditioning and further mixing to produce a homogeneous ignitible mixture takes place downstream of the inlet duct in the mixing chamber.
A flame holder arranged in accordance with the present invention is simple in design and construction and therefore relatively economical to manufacture.
In preferred embodiments of the present invention, a plurality of inventive flame holders are arranged in stellate or annular symmetry in the respective combustion area. This type of arrangement aids in the uniform heating of the main gas stream across the combustion area.
These and further objects, features and advantages of the present invention will become more obvious from the following description when taken in connection with the accompanying drawings which show, for purposes of illustration only, several embodiments in accordance with the present invention.